The thing is, if he was just doing it off from the top of his head he wouldn't just be inconsistent at the last possible second. He clearly had made up a plan for Naruto and Sakura to be romantically involved, but he didn't do it at the end which strikes everyone as extremely weird. It's not reading anything into anything, it's just plain-as-day him being contradictory with everything about the story. The Promise of a Lifetime, Sakura's confession of love, Sasuke's status as a mass-murdering criminal wanted for terrorism, absolutely none of that was reflected upon by the characters or even mentioned near the end. It would be as if George Lucas had filmed the original Star Wars movie (A New Hope) all the way up to the point where Obi-Wan is killed, and Luke and the Gang get on the Millennium Falcon and escape the Death Star only to have the movie end there abruptly only to begin again in THE SEVENTH/UPCOMING STAR WARS FILM. No mention of the Originals or Prequels, no exposition of the Force or the Jedi or Anakin's tragic past on becoming Darth Vader, no awesome six movies, just utter confusion as to what the Devil is going on.
That's how this feels. It's odd, it's dirty, and even if Kishi was rushed his editor would say "Whoa, there, what the balls is this you're handing me?! You just cut it off and make a huge friggin' time-skip?! No, I want you to end it, but end it so we can still earn money later, not by burning all your bridges!".
We're talking about the man who said that he didn't want ShikaTema to happen, who said he wasn't good at writing romance and yet gave us the beautiful relationship of MinaKushi, who said Sakura was the heroine of the story and then practically made her a side-character in 700 and The Last; consistency is clearly not his forté, if we go by this. Sure, he was consistent for 698 chapters, but what good is that if you fudge it up in the last two and then go on to make a movie to explain how what we saw in the manga's epilogue happened?
Also, I don't think he burned any bridges with the ending he wrote. After all, SP is going to add filler to the anime and apparently continue it well into 2015, when it should end on February or March, at most; The Last is hitting theaters next month, and it will sell good, because a lot of fans want closure on the story, which Kishi failed to provide in the manga. Let's not forget that a series of short-stories featuring the children that resulted from the "Great End-of-Series Pair-Off" will be made and published as books, which means Naruto will keep making money, one way or another. The people behind Naruto, and I'm not referring to Kishi, are businessmen, after all, and businessmen only want one thing: profit.
It may come off as a little bit hostile, but it is not my intention, I swear.
I know it's just a theory, but if what your saying is true then Kishi's got do alot better with story telling.
Tbh I'm actually writing a out a story for a book, and I've already got my ideas on how things will go down in the final book (third one). Including characters, plotlines, themes, development, etc.
I also want to write a book, or books, one day, and before I do that, I'll do the same as you: I'll sit down, define all the elements of the story, and only then will I be able to write it. I think Kishi either failed while doing this, or he simply didn't do it.
Some people have stated that maybe this wasn't the ending Kishi had planned all along, and that we got what we got thanks to him being pressured by someone higher in the food chain, so to speak. Due to the nature of the medium, manga are an interesting phenomenon, imo; the serialization and the whole business side to it that occurs behind-the-scenes affects the story a lot some times. Many people here have given examples of storytellers being forced to continue a story because it makes money. Hell, it's not exclusive to manga, something similar also happens in the film industry: we've got The Hobbit, which could've been done in one or two films, but they made three by adding material from other books that occurred simultaneously as the story on the book The Hobbit; we also have The Hunger Games, Twilight, and Harry Potter, which got their last books split in two movies; we got Supernatural, the TV series, which was originally planned as a 5-season show, but it was so successful that it got extended past that with a new showrunner, and is now on its 10th season; there are plenty of examples. Maybe these people are not so far from the truth, but we won't get any assurance or even a hint of this for quite some time, since Kishi won't want to burn any bridges with publications that might publish his future works (slandering your previous employer is a big no-no in most industries).
I'm of the mind that the ending was too jarring, all things considered. A part of me wants to think that this wasn't the ending Kishi intended for the series, but another part of me thinks that Kishi did. I do know for a fact that this is the canon ending of the series; whether that will be disputed in the future by Kishi himself is another story, and one that I'd like to be true, but not willing to hope for.